You can also count YouTube though it is meant only for videos. Nothing stops you from making a video out of your content. In fact good videos have a higher chance of going viral if they're interesting since people find it easier watching a video than reading content.

Reddit has the potential of turning your content viral. SO does StumbleUpon especially of you have excellent content. If you use their paid advertising and also have good content it is almost a guarantee for it going viral.

If you would like to go viral on FB or Twitter you need to be highly participative. You will also need a lot of followers, likes, retweets and so on so that your content gets noticed.

Facebook, twitter, Google plus, you tube are the most powerful social networks. Make lots of followers foor your page, add more fan pages. By going this way you have a higher chance to get your website viral.